Multiply.
Answer: When a number is next to a variable, it means they are multiplied. So, $({-1}{t^7})({-1}{t^5})$ is the same as $({-1})({t^7})({-1})({t^5})$. Now we can rearrange the factors because multiplication is commutative (a fancy way of saying that the order in which we multiply things doesn't matter). ${(-1)(-1)}{(t^7)(t^5)}$ Then simplify, and we're done! ${t^{12}}$